The Shokakus were completed in 1941 and were
barely shaken down in time to join the Pearl
Harbor Attack Force. They
were designed in 1936-1937, free from the restrictions of the naval
disarmament treaties, and they were excellent ships, with
the largest air groups of any of the carriers with which Japan started the war. Though
based on the Soryu, they were much
better protected and were completed with a much more substantial antiaircraft battery.
Their machinery was the most powerful ever installed on a Japanese
warship. Their availability may have been the tipping factor in
the decision to mount the Pearl
Harbor attack. Their chief weaknesses were an enclosed but
flimsy double hangar deck and inadequate fuel bunkerage, but this
was not obvious until after war broke out. Veteran pilots also complained
of poor habitability, with the living quarters lacking any kind of
temperature control.

The island was placed amidships to reduce
interference with long takeoff runs. The original design called
for the island to be on the port side, opposite the exhaust
trunks. However, based on landing tests carried out on Akagi, the island
was moved to the starboard side after construction had already
begun, forcing a reduction in the hangar area and addition of 100
tons of ballast on the port side. The island itself was closely
modeled on that of the Hiryu, and proved too cramped for
the larger Shokakus, forcing aircrew briefings to take
place on the flight deck rather than in the air operations center
on the second deck of the island.

The magazine protection was designed against 800 kg
(1760 lb) bombs or 8" shells while the
machinery protection was designed against 250kg (550 lb) bombs or
6" shells. In addition to their heavy overhead armor protection,
the fuel stores were surrounded by voids filled with carbon
dioxide. The hangar sides were left deliberately flimsy so that
any hangar explosion would vent to the sides without destroying
the ship. The Japanese hoped this would allow the flight deck to
remain mostly intact after a bomb hit, but this assumed that enemy
bombs would explode on impact with the hangar deck. The Americans
typically fused their bombs to explode immediately after
penetrating the flight deck, which was highly effective at tearing
up the flight deck. The underwater protection was calculated to be
53% effective against a 450kg (990 lb) charge of TNT but the
conclusion from later experiments was that this was too
optimistic.

The hangar deck was also the strength deck, and the
flight deck was made as light as possible, with eight sets of
expansion joints to accommodate hogging and sagging. The
flight deck had recesses and other provisions for two catapults,
but the Japanese never completed development of an operational
catapult design and none were ever installed on Japanese carriers.
The flight deck was equipped with ten electromagnetically-braked
arrestor wires, three crash barriers, and a wind screen. There
were two hangar decks, as with most large Japanese carrier
designs, The three lifts could be raised from the lower hanger to
the flight deck in 15 seconds, and the forward lift was large
enough to allow aircraft that had just landed to be struck below
with their wings still unfolded, in order to speed landing
operations.

The ship's battery was entirely dedicated to antiaircraft, with little
concern for dual use against surface targets. The ships were
originally equipped with three Type 94 directors around the
island, but one of these was eventually moved to the port side of
the flight deck to make room for Type 21 radar.

The high-power machinery was built around turbines
with high-pressure, intermediate-pressure, and low-pressure
sections plus cruise turbines. The cruise turbines were designed
for the rather high speed of 26 knots. The machinery seems to have
posed some manufacturing difficulty, forcing back the completion
dates of the two ships in spite of the rush to complete them
before war broke out. Shokaku was completed on 8 August
1941 and Zuikaku on 25 September 1941, giving the latter
just over two months to work up before participating in the Pearl
Harbor attack.

The Shokakus were arguably the most
successful warships in the history of the Japanese Navy. They were
certainly a considerable thorn in the side of the U.S. Navy after
Midway. It is alleged that Nimitz referred to
them as "those two ships" with the same acerbity that Robert E.
Lee referred to the Union armies as "those people."